The present invention relates to a method for producing a silicon single crystal by the Czochralski method (CZ method).
In the conventional production of silicon single crystal by the CZ method, a single crystal silicon is used as a seed crystal, and it is brought into contact with a silicon melt for seeding and slowly pulled with rotation to grow a single crystal ingot. In this operation, after the seed crystal is brought into contact with the silicon melt, the so-called necking is performed, in which the diameter of the single crystal is once decreased to about 3 mm to form a neck portion, in order to eliminate dislocations produced by propagation of slip dislocations generated in the seed crystal at a high density by thermal shock, and then the diameter of the crystal is increased to a desired diameter to pull a dislocation-free silicon single crystal. Such necking is widely known as Dash Necking method, and considered a common sense technique in pulling of a silicon single crystal ingot by the CZ method.
A conventionally used seed crystal is, for example, in a shape of cylinder or prism having a diameter or side of about 8 to 20 mm provided with a notch for setting it on a seed holder, and it has a flat surface for the bottom end, which is first brought into contact with the silicon melt. Because it must bear a large weight of the crystal ingot to pull it safely, it is difficult to further decrease the diameter of the seed crystal from the above-specified range in view of the strength of the material.
Since a seed crystal in such a shape has a large thermal capacity at its tip end to be brought into contact with the melt, a temperature difference is rapidly generated in the crystal at the instant of contact of the seed crystal with the melt and thus slip dislocations are generated at a high density. Therefore, the aforementioned necking becomes necessary in order to grow a single crystal while eliminating the slip dislocations.
However, under such a condition, the crystal must be necked at least to a minimum diameter of about 3 to 5 mm in order to make it dislocation-free, even if the necking conditions are variously controlled. But, such a diameter invites insufficient strength for supporting a single crystal ingot of which weight recently becomes heavier with use of a larger diameter of silicon single crystal, and there may be caused a problem that such a neck portion having a small diameter is broken and the single crystal ingot falls during the pulling of the single crystal ingot and so forth. For example, when the neck has a diameter of 5 mm, a crystal of about 250 kg is considered the upper limit for pulling considering safety factor, and it is difficult to efficiently produce a crystal having a large diameter.
Inventions for solving such a problem are proposed in Japanese Patent Laid-open (Kokai) Nos. 4-104988, 4-139092 and so forth. In these inventions, a tapered shape is used for a tip end portion of the seed crystal to make the thermal capacity of the tip end portion of the seed crystal to be brought into contact with the melt small so that slip dislocations to be generated at the instant of contact of the seed crystal with the melt should become very few, thereby realizing substantially necking-free crystal growth.
In these methods, the necking process can be shortened and the diameter of the neck portion can be made larger. Therefore, there can be obviated the problem that the neck portion is broken and the single crystal ingot falls during the pulling of the single crystal ingot and so forth.
A problem of such seeding methods utilizing a seed crystal having a special shape of its tip end portion is their success ratio in making a crystal dislocation-free.
That is, in these methods, if making a seed crystal dislocation-free is once failed, necking must be performed by the dash necking method, or the seed crystal must be changed to a new one and pulling must be performed again. Therefore, in order to widely use the methods for industrial purpose, it is particularly important to improve the success ratio in making a crystal dislocation-free.
However, detailed seeding conditions for obtaining sufficient reproducibility are not disclosed in the aforementioned proposed inventions, and the success ratio in making a crystal dislocation-free was not necessarily at a satisfactory level.
Therefore, the present invention was accomplished in view of such a problem of conventional techniques, and its object is to provide a method for safely and efficiently producing a silicon single crystal having a large diameter and a heavy weight with markedly improved success ratio in making a crystal dislocation-free.
In order to achieve the aforementioned object, the method for producing a silicon single crystal according to the present invention is a method for producing a silicon single crystal by the Czochralski method, in which a seed crystal having a shape of a pointed tip end or a truncated pointed tip end as a shape of its tip end portion to be brought into contact with a silicon melt is used, the tip end of the seed crystal is first carefully brought into contact with the silicon melt, then the seed crystal is descended at a low speed or a surface of the silicon melt is ascended at a low speed to melt the tip end portion of the seed crystal to a desired diameter, and subsequently the seed crystal is slowly ascended or the surface of the silicon melt is slowly descended to grow a silicon single crystal ingot without performing necking, wherein after the tip end of the seed crystal is carefully brought into contact with the silicon melt, the seed crystal is maintained at that state for 5 minutes or more to reserve heat in the seed crystal.
If the seed crystal is maintained for 5 minutes or more after the tip end of the seed crystal is carefully brought into contact with the silicon melt to reserve heat in the seed crystal as described above, the temperature of the seed crystal is sufficiently increased and therefore probability that slip dislocations are generated in the subsequent melting process can be decreased. Thus, the success ratio in making a crystal dislocation-free can be markedly improved, and a silicon single crystal having a large diameter and a heavy weight can be safely and efficiently produced.
Further, the method for producing a silicon single crystal according to the present invention is also a method for producing a silicon single crystal by the Czochralski method, in which a seed crystal having a shape of a pointed tip end or a truncated pointed tip end as a shape of its tip end portion to be brought into contact with a silicon melt is used, the tip end of the seed crystal is first carefully brought into contact with the silicon melt, then the seed crystal is descended at a low speed or a surface of the silicon melt is ascended at a low speed to melt the tip end portion of the seed crystal to a desired diameter, and subsequently the seed crystal is slowly ascended or the surface of the silicon melt is slowly descended to grow a silicon single crystal ingot without performing necking, wherein the tip end of the seed crystal is carefully brought into contact with the silicon melt to melt the tip end portion of the seed crystal for a length of 5 mm or less, and then the seed crystal is maintained for 5 minutes or more to reserve heat in the seed crystal.
It was found that, if the tip end of the seed crystal is carefully brought into contact with the silicon melt to melt the tip end of the seed crystal for a length of 5 mm or less, and then the seed crystal is maintained at that state for 5 minutes or more as described above, the temperature of tip end portion of the seed crystal can be made approximately equal to the temperature of the silicon melt surface. Further, it was found that, since the tip end portion of the seed crystal is melt at instant of contact with the silicon melt thanks to the use of a pointed shape of the tip end so long as the melted length is 5 mm or less, slip dislocations are not generated.
In the steps of melting the tip end portion of the seed crystal to a desired diameter and then slowly ascending the seed crystal or slowly descending the silicon melt surface to grow a single crystal ingot in these methods, it is preferred that the diameter of the crystal is decreased by 0.3 mm or more but 2 mm or less from a diameter at the time when the melting is finished and then the diameter is increased within a section of at least 3 mm from the position at which the melting of the tip end portion of the seed crystal to the desired diameter is finished.
If the temperature of the silicon melt is controlled so that the diameter of the crystal is decreased by 0.3 mm or more but 2 mm or less from a diameter at the time of finishing the melting and then the diameter is increased within a section of at least 3 mm from the position at which the melting of the tip end portion of the seed crystal to the desired diameter is finished in the steps of slowly ascending the seed crystal or slowly descending the silicon melt surface to grow a single crystal as described above, the tip end portion of the seed crystal should have a temperature at which it is rapidly melted in the step of melting the seed crystal and therefore the seed crystal can be melted to the desired diameter without generating slip dislocations.
The above characteristics are employed for the following reasons. When a crystal having a large diameter and a heavy weight is produced, the amount of silicon melt in a crucible also becomes to have a heavy weight and hence a large thermal capacity and thus the temperature of the silicon melt cannot be lowered in a short period of time. Therefore, in order to increase the diameter immediately after finishing the melting, decrease of the silicon melt temperature must be started during the step of melting the seed crystal. However, when the silicon melt does not have a sufficiently high temperature, the tip end of the seed crystal cannot be rapidly melted at the melt surface and thus it sinks into the silicon melt in the state of solid. Thus, slip dislocations are generated. If the melting is performed to a sufficient diameter without generating slip dislocation by the above method, a sufficient diameter can be secured in the subsequent diameter increasing step even after the diameter is decreased by 0.3 mm or more but 2 mm or less within the section of 3 mm after the melting is finished. Thus, strength does not become insufficient even when a crystal having a large diameter and a heavy weight is produced.
Furthermore, in these methods, as for the rate of melting the tip end portion of the seed crystal to a desired diameter by descending the seed crystal at a low speed or ascending the silicon melt surface at a low speed, the descending speed of the seed crystal or the ascending speed of the silicon melt surface can be determined as a speed changing continuously or stepwise so that the volume of the tip end portion of the seed crystal melting per minute should become 50 mm3 or less.
In order to melt the tip end portion of the seed crystal without generating slip dislocations, such conditions can be selected that the tip end portion of the seed crystal should be rapidly melted at the melt surface and thus it should be prevented from sinking into the silicon melt in the state of solid. However, if the conditions are adjusted only by lowering the rate of melting the tip end portion of the seed crystal, time required for the melting becomes longer. Thus, the productivity is degraded and in addition, slip dislocations may also be generated due to temperature fluctuation of the silicon melt contacting with the tip end portion of the seed crystal.
Furthermore, in these methods, a horizontal magnetic field of 1000 G or more is preferably applied to the melt surface in a crucible accommodating the silicon melt.
If a horizontal magnetic field of 1000 G or more is applied to the melt surface in the crucible accommodating the silicon melt, convection of the silicon melt is suppressed, and the generation of slip dislocations due to temperature fluctuation of the melt can be avoided. Therefore, the success ratio in making a crystal dislocation-free can be markedly improved.
As explained above, according to the present invention, a high success ratio in making a crystal dislocation-free can be attained in the dislocation-free seeding method in which seeding is performed without necking, and it enables production of dislocation-free silicon single crystals having a heavy weight with higher productivity, higher yield and lower cost compared with conventional methods.